Solvent, metal and halogen-free synthesis of sulfoxides by using a recoverable heterogeneous urea-hydrogen peroxide silica-based oxidative catalytic system

Article abstract of DOI:10.1016/j.catcom.2017.06.014

An efficient and green protocol was used for the selective oxidation of sulfides to sulfoxides by using silica sulfuric acid (SSA) as a recoverable heterogeneous solid acid catalyst in the presence of urea-hydrogen peroxide (UHP) under toxic transition me

Full text of DOI:10.1016/j.catcom.2017.06.014

Accepted Manuscript
Solvent, metal and halogen-free synthesis of sulfoxides by using
a recoverable heterogeneous urea-hydrogen peroxide silica-based
oxidative catalytic system
Mojtaba Azizi, Ali Maleki, Farahman Hakimpoor
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DOI:
Reference:
S1566-7367(17)30245-5
doi: 10.1016/j.catcom.2017.06.014
CATCOM 5080
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13 June 2017
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metal and halogen-free synthesis of sulfoxides by using a recoverable heterogeneous
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ACCEPTED MANUSCRIPT
Solvent, metal and halogen-free synthesis of sulfoxides by using a recoverable
heterogeneous urea-hydrogenperoxide silica-based oxidative catalytic system
Mojtaba Azizia,b, Ali Maleki,a,* maleki@iust.ac.ir Farahman Hakimpoorb
aCatalysts and Organic Synthesis Research Laboratory,
Department of Chemistry,
Iran University of Science and Technology Tehran,
1684613114, Iran
bDepartment of Chemistry,
Faculty of Science,
Lorestan University,
Khoramabad, Iran
*Corresponding author.
Abstract
An efficient and green protocol was used for the selective oxidation of sulfides to sulfoxides by
using silica sulfuric acid (SSA) as a recoverable heterogeneous solid acid catalyst in the presence
of urea-hydrogen peroxide (UHP) under toxic transition metal-, solvent- and halogen-free
conditions at room temperature. The present method offers several noteworthy advantages, such
as high product yields, easy work-up, non-toxicity, easy preparation and reusability of the
catalyst for at least six runs, which make this procedure the most convenient and attractive with
low cost of catalyst and oxidant.
Keywords: UHP/silica sulfuric acid; Heterogeneous catalyst; Oxidation; Sulfides; Green
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chemistry.
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1. Introduction
The selective oxidation of organosulfur compounds such as sulfides to the corresponding
sulfoxides has been a challenge for many years owing to the extensive application of sulfoxides
in organic synthesis and their importance in various medical, biological and technological
applications, including: drugs [1-3], flavors [4], germicides [5], chiral ligands [6,7] and
molecular rearrangements [8,9].
For this reason, many methods have been developed over the years to increase the
efficiency of these kinds of oxidation reactions. For example, oxidation of sulfides has been
reported in the presence of various catalysts and oxidants (e.g. metal oxidants, organic oxidants,
peroxides, halogens and air) [10]. Among a lot of different oxidants, aqueous hydrogen peroxide
has been one of the most important ‘‘green oxidants’’, offering the advantages of safety in
storage and operation, being cheap, readily available, having a highly effective oxygen content
and environmentally-benign with the formation of water as the only by-product [11-15].
Although different catalysts, such as polyoxometallates and transition-metal Schiff-base
complexes, have been used for H2O2-based oxidation of sulfides, most of them suffer from one
or more limitations such as high cost, the requirement of a promoter or co-catalyst, high
temperatures, long reaction times, chlorohydrocarbon solvents and excessive use of H2O2.
Furthermore, there are several disadvantages for the metal-catalyzed methods such as the use of
toxic transition metal compounds, the preparation of complex catalysts and remaining metals in
products [16-20]. In spite of all the advantages of H2O2, the handling of its concentrated
solutions is not desirable because of safety problems. Therefore, urea-hydrogen peroxide (UHP),
as an urea-adduct compound has attracted much attention since it is inexpensive, commercially
available, stable solid and anhydrous source of H2O2 [21]. In addition, most of the catalysts are
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required to use a solvent in the reaction, while it is highly favorable to expand a process in the
absence of any solvent, which this aspect is very important from the viewpoint of green
chemistry.
Due to environmental, economic and industrial aspects and based on green chemistry
principles, recoverable heterogeneous catalysts play important roles in the development of
organic functional group transformations and are suitable alternatives for transition metals and
liquid acid catalysts [22-28]. Owing to the numerous privilege of inexpensive and non-hazardous
reusable heterogeneous catalysts, silica sulfuric acid (SSA) has been considered as a powerful
catalyst for a variety of organic reactions under facile circumstances [27, 28].
2. Experimental
2.1. General
All chemicals were purchased from Merck and Aldrich chemical companies. Melting
points were measured on a Buchi B-540 apparatus. Infrared spectra were recorded on ABB
1
FTLA 2000 Bruker FT-IR spectrophotometer. H and 13C NMR spectra were recorded in CDCl3
as the solvent and TMS as the internal standard on a Bruker AQS 300 Avance spectrophotometer
at 300 and 75 MHz, respectively. Preparative TLC was performed using silica gel kieselgel 60
PF254+366. All yields refer to isolated products.
2.2. Preparation of silica sulfuric acid (SSA)
A 500 mL suction flask was equipped with a constant pressure-dropping funnel
containing chlorosulfonic acid (23.3 g, 0.2 mol) and a gas inlet tube for conducting HCl gas over
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an absorbing solution, i.e. H2O. Then, 60.0 g of silica gel was charged into the flask.
Chlorosulfonic acid was added drop wise over a period of 30 min at room temperature. HCl gas
evolved from the reaction vessel immediately. After the addition was complete, the mixture was
shaken for 30 min and SSA was obtained (76 g) as a white solid (Scheme 1) [27]. Synthesized
catalyst was characterized with FT-IR spectroscopy.
Scheme 1. Preparation of SSA.
2.3. General procedure for the oxidation reactions catalyzed by UHP/SSA
The solid reaction mixture containing the sulfide (1 mmol), UHP (2 mmol) and SSA (0.1
g) were thoroughly mixed and the resulting mixture was stirred magnetically. The progress of the
reaction was followed by TLC (eluent: n-hexane/ethyl acetate: 2:1). After completion of the
reaction, CHCl3 (20 mL) was added and the solid catalyst was removed by filtration. The filtrate
was washed with water (15 mL) and the extract was dried over MgSO4. After evaporation of the
solvent, the crude product was purified by chromatography on silica gel (25–30 mesh), eluting
with ethyl acetate/n-hexane to give pure sulfoxides.
Spectral data of a selected product
o
o
4-(Bromo phenyl) benzyl sufoxide: Colorless crystals. M.p.: 138-139 C (139-140 C) [32]. IR
-1
1
(KBr) ν: 3060, 2961, 1454, 1067, 1038, 766 cm . H NMR (300 MHz, CDCl3) δ: 4.01 (d, 1H, J
= 12.4 Hz), 4.1 (d, 1H, J = 12.4 Hz), 6.96-6.99 (d, 2H, J = 6 Hz), 7.18-7.33 (m, 5H), 7.52-7.56
(d, 2H, J = 8.4 Hz). 13C NMR (75 MHz, CDCl3) δ: 63.44, 125.62, 126.03, 128.43, 128.55,
128.63, 130.36, 132.02, 141.89.
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3. Results and Discussion
Current demand for environmentally friendly processes requires the synthesis of new catalysts
and the development of green oxidation methods that use clean oxidants, such as hydrogen
peroxide, without any solvent. Because of the importance of solid-phase-assisted reaction
systems in green chemistry, as well as to overcome the above-mentioned limitations, such as
over-oxidation to sulfones, low selectivity, low yields, toxicity and expensive reagents or
catalysts, and also as part of our continued research on the development of new routes for
oxidation reactions [29] and also introducing urea-hydrogen peroxide (UHP) as an ideal green
reagent in organic synthesis, we have used SSA in the no-solvent oxidation of sulfides with
UHP as a green oxidant. This process is clean, safe, selective, simple and cost-effective for the
oxidation of sulfides to the corresponding sulfoxides.
In this work, we developed a simple, efficient and environmentally benign method for the
selective oxidation of sulfides to their corresponding sulfoxides with UHP in the presence of
SSA as a reusable heterogeneous catalyst. SSA is easily prepared from the treatment of silica gel
with chlorosulfonic acid at room temperature. It must be noted that the preparation method is
simple, clean and without workup procedure since HCl gas is evolved from the reaction vessel
immediately. The notable advantages of the current method are: operational simplicity, cheap
and chemically stable oxidant and catalyst, good to high yields of products and short reaction
times. More importantly, the catalysts can be easily recovered using simple filtration and reused
for several times without significant loss of their catalytic activity. Therefore, the oxidation
method meets the needs of contemporary “green chemistry” and is suitable for practical
synthesis.
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3.1. Characterization of silica sulfuric acid (SSA)
The prepared silica sulfuric acid (SSA) was characterized by FT-IR spectroscopy (Fig.
1). For silica (SiO2), the major peaks are broad anti-symmetric Si–O–Si stretching from 1000-
-1
-1
1200 cm and symmetric Si–O–Si stretching near 850 cm . For the sulfonic acid functional
-1
group, the O-S-O stretching absorption modes are observed at 1100–1170 cm and the S–O
-1
stretching at 680 cm . FT-IR spectrum shows an overlap between asymmetric and symmetric
stretching bands of SO2 and Si–O–Si stretching bands in the silica sulfuric acid. A broad OH
-1
stretching appears at 3440 cm .
Figure 1. FT-IR spectra of silica sulfuric acid (a) and silica gel (b).
3.2. Optimization of the reaction conditions
To optimize the reaction conditions, 4-(bromo phenyl) benzyl sulfide was selected as a
model substrate and the relative amount of the catalyst and oxidant were investigated. The
detailed results obtained are described below.
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3.2.1. Optimization of the amounts of the catalyst and oxidant
In general, hydrogen peroxide adducts are not able to oxidize organic compounds by
themselves; therefore, introduction of an inorganic catalyst or organic mediator for active oxygen
transfer of these adducts is a necessity in any oxidation systems. As expected, control
experiments indicated that the reaction did not occur in the presence of UHP without catalyst,
result that confirms the essential role of SSA in the reaction. Then, similar reactions were studied
in the presence of various amounts of SSA and UHP. As shown in Table 1, an optimum ratio of
4-(bromophenyl) benzyl sulfide (1 mmol)/UHP (2 mmol)/SSA (0.1 g) was found for complete
conversion of sulfides to sulfoxides. Higher amounts of SSA did not have any effect on the
completion and efficiency of the reaction.
a
Table 1. Optimization of the relative amounts of reagents under solvent-free conditions.
Entry
UHP (mmol)
SSA (g)
0.05
0.1
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
1
1
300
120
90
33
40
52
65
85
95
95
95
1
0.15
0.05
0.1
1.5
1.5
2
90
65
0.1
35
2
0.15
0.1
40
2.5
40
a 4-(Bromo phenyl) benzyl sulfide (1 mmol), r.t.
b Isolated yields.
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3.3. Application of SSA/UHP catalytic system for the oxidation of various sulfides
In order to investigate the scope and limitations of the present protocol and to illustrate
the application of the SSA/UHP system in the synthesis of sulfoxides, a variety of products were
synthesized under the optimized conditions. The results are summarized in Table 2. It is shown
that all the sulfoxide products were obtained selectively in good-to-excellent yields after
appropriate reaction times, and no over oxidation products were detected in the reaction
mixtures.
Furthermore, to screen the applicability of the present methodology, a comparison with
some of previously reported catalysts was provided in the oxidation of methylphenyl sulfide (see
Table S1 in Supporting Information file). All the products were known compounds, which were
identified by comparison of their melting points with those authentic literature samples. They
1
13
have also characterized by FT-IR, H NMR and C NMR spectroscopies.
As shown in Fig. 2a, according to the FT-IR spectrum, the presence of a strong
-1
absorption band at near 1038 cm is related to S=O stretching vibration that approved the
1
formation of 4-(bromo phenyl) benzyl sulfoxide as a sole product. In addition, H NMR
spectrum (Fig. 2b) clearly confirmed the successful conversion of 4-(bromo phenyl) benzyl
sulfide to 4-(bromo phenyl) benzyl sulfoxide.
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Table 2. Oxidation of sulfides to sulfoxides in the presence of SSA/UHP.a
M.p.( °C)
Time
(min)
Yielda
(%)
Entry
R1
R2
Found
Reported
124 [30]
1
2
3
Ph
PhCH2
PhCH2
PhCH2
35
40
35
95
93
95
122-123
127-129
138-139
4-Cl-C6H4
4-Br-C6H4
126-127 [31]
139-140 [32]
4
5
n-Octyl
Ph
PhCH2
Ph
35
65
90
69
69-70
71-72
71-71.5 [33]
72 [30]
6
7
8
9
4-Me-C6H4
PhCH2
PhCH2
PhCH2
CH3
40
30
35
30
93
95
91
88
122-124
135-136
121-122 [31]
136 [30]
4-Cl-C6H4
n-Butyl
42-44
45-46 [30]
Oil [31]
n-Butyl
Colorless oil
10
11
12
Ph
CH3
CH3
30
35
35
96
94
94
Colorless oil
74-76
Oil [31]
72-75 [34]
138-139 [20]
4-Br-C6H4
4-Br-PhCH2
PhCH2
139-140
13
14
Ph
Allyl
30
25
95
93
Colorless oil
Colorless oil
Oil [30]
Oil [35]
a Isolated yields.
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b
a
1
Figure 2. FT-IR (a) and H NMR (b) spectra of 4-(bromo phenyl) benzyl sulfoxide.
3.4. Catalyst recycling
In order to assess the stability of the catalyst after completion of reaction, it was separated
from the reaction mixture by filtrations, washed with the solvent (CHCl3) and dried at 60 °C.
From Fig. 3, it can be observed that the catalyst could be reused at least six times with
conversions of benzyl phenyl sulfide to the corresponding sulfoxide decreasing by only 11.5%.
Figure 3. Recycling of SSA catalyst in the synthesis of 4-(bromophenyl)benzyl sulfoxide.
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4. Conclusions
In summary, we have developed and tested an efficient, green and environmentally-
friendly protocol for the selective oxidation of sulfides to sulfoxides under toxic transition metal-
, solvent- and halogen-free conditions. This protocol offers several noteworthy advantages,
namely: high yields of products, easy work-up, sustainability, non-toxicity, easy preparation,
room temperature and recyclability and low cost of catalyst and oxidant used.
Acknowledgments
Financial supports by the Research Councils of the Iran University of Science and Technology
and Lorestan University gratefully acknowledged.
Supporting information
1
Additional supplementary data including full spectral data, copies of FT-IR, H NMR and 13C
NMR spectra of the products as well as physical properties are available in the online version.
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Graphical Abstract
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Highlights
 A mild and green route for the oxidation of sulfides to sulfoxides was introduced.
 A metal, solvent and halogen-free protocol was reported.
 Sulfoxides were observed as the sole oxidation products in high yields.
 Recoverable heterogeneous solid acid catalyst can be reused for several times.
1
13
 The products characterized by FT-IR, H NMR and C NMR measurements.
17